RE: creating virtual user a/c and domains in sendmail

Hi,
The doc sendmail.org/vitrual speaks of creating a virtualuser table
and mappimg the useraccounts.
the command refered is
$makemap dbm /etc/mail/virtusertable < sourcefile
where sourcefile is a flat file containing the mappings.
1. it says dbm not supported in my sendmail, why?
2. say a useracct "test"
can have two domains, say
test@xxxxxxxxxxxxxx
test@xxxxxxxxxxxx
test@xxxxxxxxxxxx
how does the system diffrentiate the mail
regards,
vinod